The present invention relates to a semiconductor device and in particular to a semiconductor device having an MOS type structure suitable for dealing with the low supply voltage foreseen at the epoch of deep sub-.mu.m.
The trial of making semiconductor devices finer and finer is at the epoch of sub-.mu.m and it is realized that products of level of 0.8 .mu.m rule are produced in mass. The tendency of making semiconductor devices finer will be advanced further and it is thought to be sure that it is advanced up to a level of 0.1 .mu.m rule. However, on the other hand, when the trial of making them finer is advanced as described above, the breakdown voltage of MOS transistors of actual structure is lowered, and thus it is not possible to maintain the actual operation with a supply voltage of 5V. Therefore, it has been determined that the lowering in the supply voltage is necessary. Although this lowering in the supply voltage alleviates deteriorations in characteristics due to hot carriers, on the other hand it leads to a lowering of the driving current, and thus the propagation delay time becomes longer. In order to improve it, it is important to increase the driving current of MOS transistors and to decrease the capacitance parasitically added such as the capacitance of wiring, the junction capacitance of the source and the drain of MOS transistors, etc. as far as possible.
A method for decreasing the junction area of the source and the drain to reduce the junction capacitance for achieving such an object is disclosed in JP-A-63-283065, JP-A-62-94985, etc.
Hereinbelow the outline of this method will be explained, referring to FIGS. 1A to 1C. At first, a thick oxide film 2 called field oxide film is formed outside of a region, where an element is to be formed, by the well-known local oxidation (LOCOS) method. Then a gate oxide film 10 is formed in the region, where the element is to be formed, and a gate electrode 3 is formed thereon. In order to give this gate oxide film an anti-oxidation property, an oxide film 21 and an Si.sub.3 N.sub.4 film 22 are superposed thereon (FIG. 1A). A spacer 35 made of Si.sub.3 N.sub.4 is formed around this gate electrode and a first field oxide film 5 is formed by using this spacer 35 as a mask (FIG. 1B). By this oxidation the parts acting as the source and the drain are the only parts, which are covered by the spacer (Si.sub.3 N.sub.4) 35, so that these regions can have an extremely small width of 0.2 to 0.5 .mu.m. Thereafter the spacer made of Si.sub.3 N.sub.4 is removed and the new spacer made of SiO.sub.2 is formed to make the conventional LDD (Lightly Doped Drain) structure (FIG. 1C).
A feature of this technique consists in that the width of the source and the drain is determined by the width of the spacer 35 made of Si.sub.3 N.sub.4, which is useful for decreasing the area of the source and the drain, leading, of course, to a decrease in the parasitic capacitance.
However, in the prior art technique, no attention is paid to the increase in the driving current. That is, there was a problem that no measures are taken against the decrease in the channel width due to bird's beaks (intrusion of an oxide film produced at the edge portion of a film of anti-oxidation property) grown in the formation of the field oxide film 2 and that the channel width at the completion is smaller than that foreseen at the design.